This invention relates to electrical resistance heaters, and in particular to the use of such heaters in microcircuit substrate and solid state applications.
Electrical resistive heaters are used on microcircuit substrates to heat various elements and are also used with silicon photodetectors to maintain an appropriate temperature for optimum detection operations. In either instance rapid response and uniform heating characteristics are desirable. In the case of silicon photodetectors and in certain microcircuit applications it is also necessary that fringing magnetic fields and their concommitant inductive interactions be elminated. For instance, since the sensitivity of the photodetector is limited by electrical noise the heater must be designed to eliminate any such noise contribution due to currents induced by turning the heater on and off. Conventional resistance heaters used in these applications commonly have geometric configurations that induce fringing magnetic fields. The present invention is directed toward providing a heater that overcomes this deficiency and that further exhibits rapid response and uniform heating characteristics.
The annular shape of the resistance element of this invention has an additional advantage over the usual narrow width or long serpentine shape of prior art resistive heating elements. Thin film and thick film resistors, including cermet type, can degrade during use by developing fine cracks, especially as a result of many fast temperature changes or on-off cycling. One such crack in a narrow width resistor can easily extend completely across the small dimension and result in an open circuit. The annular shape of the disclosed resistor permits small cracks to occur without resulting in an open circuit.